With the development of increasingly large wind turbines, there is a requirement for wind turbine blades of greater length. However, the use of longer blades poses a number of problems. One such problem is that as the blades get longer, the flatwise bending of the tip of the blade during use increases and there is therefore a greater risk of the blade tip colliding with the tower of the wind turbine during high winds. In order to ensure that there is sufficient clearance between the blades and the tower at all times, it is necessary either to mount the blades at a greater distance from the turbine tower, or alternatively, to increase the stiffness of the blade tips such that the deflection is reduced. The latter of these solutions is preferred since the first solution is known to increase the costs of the wind turbine as a whole.
It has also been recognised that it is advantageous to reduce the tip chord of wind turbine blades as much as possible. Blades with a narrower tip chord offer a number of advantages over standard blades, including reduced loads and cost for the rest of the turbine. However, the problem of tip deflection, as described above, becomes greater as the tip portion of the blade gets narrower. The larger part of the tip deflection is generated in the outer part of the blade towards the tip end where there is less material. Blades with a reduced tip chord are therefore only viable if they can be formed from materials having a sufficiently high level of stiffness, to compensate for the reduced stiffness of the blade tip portion arising from the smaller cross-section. It is generally more economically viable to stiffen the blade towards the tip end, where there is less material.
Conventionally, composite materials reinforced with glass fibres have been used to produce wind turbine blades. However, to obtain the higher level of stiffness required for longer and/or narrower blades, increased amounts of glass fibres are required. This results in a heavier and less efficient blade.
Carbon fibres are also used in composite materials for use in wind turbine blades despite their higher raw material costs, since they are lighter and stiffer than glass fibres and therefore offer improved reinforcement. WO-A-03/078832 and WO-A-03/078833 both disclose a wind turbine blade having a tip end portion made substantially from carbon fibre-reinforced polymer and a root end portion made substantially from glass fibre-reinforced polymer. Also, EP 1 746 284 discloses a wind turbine blade in which the outer shell halves are reinforced by the inclusion of pre-fabricated strips of carbon fibres.
It would be desirable to provide an improved wind turbine blade, which is formed from a material that optimises the strength and stiffness of the blade, such that the deflection of the tip end of the blade can be reduced. It would be particularly desirable to provide a blade with sufficient stiffness at the tip that the tip chord could be significantly reduced, without the problem of tip deflection. It would also be desirable to provide a wind turbine blade of increased length compared to standard blades, which is both efficient and cost effective.